With the increasing number of users of communication networks, such as the Internet, and with increases in capacity and variety of content, such as high-definition images, an increase in transmission capacity is desired in radio communication as well. As for a large-capacity radio transmission technique, the use of a millimeter-wave band has been considered which is not used by many commercial radio stations and in which a wide frequency band is readily ensured. Accordingly, attention has been paid to modulated impulse radio communication using a modulated impulse that is a signal whose pulse width is very small (a signal exhibiting an instantaneous increase and decrease in voltage).
A radio communication apparatus used for the modulated impulse radio communication does not necessarily have to include a high-precision local oscillation unit and a mixer in comparison to a narrow-band communication apparatus using a continuous carrier scheme. It is expected that such a radio communication apparatus will be an apparatus which allows a radio unit thereof to have a simple configuration and be provided at low cost and which allows large-capacity radio transmission of over 10 gigabits per second (10 Gbps) using a wide bandwidth.
A radio signal used in modulated impulse communication is a bandpass signal having a wide band with respect to a center frequency and two basic principles of generating a modulated impulse modulated wave will be exemplified below.
1. A modulated impulse modulated signal is obtained by modulating a local signal with a return to zero (RZ) data signal having a short pulse width as in the case of typical continuous wave communication.
2. A local oscillation unit is operated only during a time period over which an RZ data signal is at a high level.
In an existing radio communication apparatus, it takes time before an oscillation output level rises. Hence, power has to be supplied for a long time even when a single modulated impulse is generated, and it is difficult to effectively reduce power consumption.
When the first principle is used, a high-precision modulated impulse modulated wave may be generated; however, even when all circuits do not transmit modulated impulses, the circuits' still have to operate. When the second principle is used, the oscillation unit is stopped when a modulated impulse is not transmitted; however, it takes a long time for oscillation to start before a modulated impulse is then transmitted, thereby making it difficult to provide high-speed communication.
More realistic modulated impulse radio communication techniques, such as the following techniques, have been suggested. For example, there is a technique of the related art in which a modulated impulse is generated on the basis of a data signal of 10 Gbps input from a baseband unit and a frequency component used for communication is extracted from among wide band frequency components contained in the generated modulated impulse by using a band-limited filter.
There is another technique of the related art in which a bias of a delay-locked loop (DLL) is adjusted so as to be locked to a reference signal and sampling clocks having twice the desired carrier frequency are thereby generated, and transmission signals sequentially inverted using the sampling clocks are sampled to thereby generate a modulated impulse modulated wave.
However, in the technique of the related art using a band-limited filter, passage through the band-limited filter increases a time width of a modulated impulse and limits a modulation rate for communicating large volumes of high-speed data.
Furthermore, in the technique of the related art in which a carrier frequency is externally adjusted by using a reference signal and transmission data sequentially inverted by using generated sampling clocks is sampled to thereby generate a modulated impulse modulated wave, only a modulated impulse modulated wave having a frequency at which a sampling circuit operates, for example, about 2 GHz as an upper limit, is generated, and a modulation rate is less than or equal to a fraction of this frequency.
Examples of the related art include Japanese Laid-open Patent Publication No. 2008-205733 and Japanese Laid-open Patent Publication No. 2008-288888.